The present invention relates to a display device for image display by using a backlight and a liquid crystal display in combination.
A liquid crystal display as a display device is configured by combining a backlight and a liquid crystal panel. This backlight illuminates the liquid crystal panel in its whole area or in multiple divided segments. The liquid crystal panel has a structure having arranged in a plane a number of pixels with a function of transmittance control (or reflectance control) by liquid crystal elements, each pixel being provided with a color filter. As the liquid crystal panel is combined with a backlight, the liquid crystal panel becomes a display device capable of displaying color images.
The basic requirement of the backlight is to illuminate the liquid crystal panel uniformly, and the light emission characteristics, which contribute to uniform lighting, include wavelength distribution, luminance, full-width half maximum, and dominant wavelength. If some characteristics are not uniform, the rays incident on the liquid crystal panel are not uniform, and rays output from the liquid crystal panel under a control become irregular, resulting in deterioration of quality of the displayed image.
For example, when a fluorescent lamp is used as a backlight source, a fluorescent lamp has its light uniformity improved by a combined use of a white-light fluorescent lamp in a length close to the screen size and a scatter plate for optically scattering light rays emitted by the fluorescent lamp. Because a fluorescent lamp can be approximated by a line light source and its light emission is converted into a surface light source, a spatial passage or a volumetric capacity for mixing light rays is indispensable for the fluorescent lamp.
Recently, with the improvement in the performance of semiconductor light emitting devices, attempts have been made to use semiconductor light emitting devices as a light source for the backlight. Among semiconductor light emitting devices, there are LEDs (light emitting diodes) and LDs (laser diodes). Those semiconductor devices, such as LEDs and LDs, are different in properties from conventional fluorescent lamps in that an LED or an LD has a precipitous rise in their light emission wavelength distribution and that the LED or LD can be approximated by a point light source (the semiconductor chip size is small).
To use LEDs, which are point light sources, as a surface-light-source backlight, it is necessary to obtain wider scattering of light by LEDs than by a fluorescent lamp. If it is impossible to provide sufficient scattering of light, irregularity occurs on an image. When forming a backlight by arranging a large number of LED devices in one plane, it ought to be noted that the variation in characteristics among the devices and the irregularity caused by the optical structure are the factors that deteriorate display quality.
To suppress irregularity such as described, the use of a scatter plate to mix the light rays from the light emitting devices is effective; however, this contributes to an increase in volume of the device because it is necessary to secure an optical path for the light rays. To minimize the variation in characteristics among devices, it is effective to sort devices but this takes sorting instrument and time.
Shinpen Shikisai Kagaku (New-Edition Color Science) Handbook 2nd Edition (compiled by The Color Science Association of Japan, published 1998/06 by Tokyo University Press) describes a method by which colors perceived by human visual sense are expressed by color signals in numeric form and also a method by which the irregularity in a displayed image on a display device is corrected by using color signals. This Handbook describes in detail the CIE 1931 XYZ calorimetric system established by CIE (International Commission on Illumination) in 1931 as a method for numerically quantifying colors by three kinds of color signals X, Y and Z based on human visual sense characteristics.
It is known that the human visual sense characteristics recognize a color image by a combination of color signals having at least three kinds of wavelength distribution, and that as the three kinds of color signals, red, green, and blue (RGB), or hue, saturation, and luminance (HSL), or XYZ are used.
The XYZ calorimetric system is a method for numerically expressing colors based on the human visual sense characteristics, and by this method, the visual sense characteristics expressed by three kinds of spectrum distribution can be replaced by three values X, Y and Z. By calculating chromaticity values, such as x y (low-case x and y) based on XYZ values, colors can be expressed numerically.
By using appropriate conversion equations, RGB or HSL are converted into XYZ signals. With any calorimetric system, at least three kinds of color signals are required to express colors based on human visual sense.
There has been proposed a method for realizing a uniform display quality on a displayed image on liquid crystal panel configured to control transmittance of light received from the backlight by adjusting display signals for transmittance control.
JP-A-8-313879 reveals a method for correcting irregular display factors of a display device by signal processing, the method having been developed with sights set on two characteristics, that is, the luminance and the hue on the display image.
However, the colors that the human eye perceives are represented by three kinds of signals as shown in the Color Science Handbook mentioned above. Therefore, if only the two kinds of characteristics are addressed in coping with irregular display image, it follows that one dimension of the human visual sense characteristics is missing. For example, in the three-dimensional calorimetric system of hue, saturation, and luminance (HSL), if coordinates are luminance and hue only, a coordinate for saturation is ignored here.
Problems to be solved by the present invention are described below. Firstly, when semiconductor light emitting devices are used as backlight sources, such as LEDs for example, since the LEDs may be referred to as point light sources if compared with a fluorescent lamp, their light quantity distribution varies notably. Among the individual LEDs, there are variations in characteristics, such as the peak wavelength (dominant wavelength) or the full width at half maximum of the emission wavelength distribution of the LED. Those variations give rise to differences of primary colors of the illumination, generating irregular color on a displayed image. If there is variation in the emission wavelength distribution (spectrum) of the LED, so long as only the luminance and the hue are used as correcting objects, sufficient correction cannot be obtained and irregular color cannot be eliminated.
Secondly, if one takes note of characteristics of signals supplied to a displayed image which is to be the target after correction has been made, generally, the center area of the displayed image tends to be light and the peripheral region dark for reasons of the optical structure. With the visual sense of a human being, we often gaze at the center area, so that it is desirable that the center area is lighter than the peripheral area. Despite this, if signals are corrected to make the luminance uniform over the whole displayed image, the signal correction process will take place to reduce the brightness of the center area in accordance with the darkness of the peripheral area. This suppresses the lighting unit's fundamental capacity of providing the brightness of the center area of the displayed image.